do_usage: use argv[0] a name, simplify opts
do_usage: use argv[0] a name, simplify opts

For issue #210, make sure we report the binary name as called, to avoid
any inconsistencies.  Since the invocation line wasn't up-to-date for
ages, just reduced it to the only mandatory argument.  This doesn't
change the default output binary name in order not to break all
downstream packaging.

grobian/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,piotr1212/carbon-c-relay,grobian/carbon-c-relay,grobian/carbon-c-relay